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Processing | electrical and electronics machines.

Germany-based Ferromatik Milacron developed a similar system, which it marketed for some time as Barrel Compression Injection (BCI) moulding. “We made a lot of tests and saw some benefits but we sold less than 10 systems,” says Ferromatik Milacron director of R&D Thorsten Thümen. The company no longer actively markets the BCI

technology, he says, but offers an alternative – High Precision Moulding (HiPM) – that builds on some of the observed benefits. “We are now using the compression at the beginning of the process just to give enough pressure to shut the non-return valve,” says Thümen. The system does not provide a high level of pressuri- sation – typically no more than 100bar is generated in front of the non-return valve – but it does ensure positive activation of the check ring. In precision moulding applications, inconsistent activation of the non-return valve can introduce considerable shot variation, according to Ferromatik. The HiPM technique is a software option available on

the company’s all-electric machines and functions with standard nozzles and non-return valve. It enables the user to be absolutely certain that the non-return valve has close completely before injection is commenced. In that way, delivering an accurate shot becomes a matter of simply programming the required injection screw stroke. The stroke itself does not vary. However, its position can.

Thümen says the system has proved very effective

for production of parts such as electrical connectors or demanding small medical components where delivering a constant shot volume is vital. However, convincing users can be quite a challenge as – just as with melt expansion technology – it does mean adopting non- traditional moulding methods. “The user defines the distance he wants to move the

screw and the machine delivers the full stroke. That stroke is constant but it can move so it is not possible to use the screw cushion value as a quality indicator,” says Thümen. He says the company has more than 20 customers using the HiPM moulding technology. The different flow dynamics resulting

from the use of expansion injection moulding – which by initiating at very high speeds and pressures that slowly diminish is almost the inverse of traditional injection moulding – has been considered by many to be likely to result in different properties in the final part. Last year, a technical paper was published by Professor Dietmar Drummer, head of the LKT

institute of polymer technology at the University of Erlangen in Nürnberg in Germany, and institute co- worker Karoline Vetter that took a detailed look at this area. Earlier work by the same team has shown that expansion moulding does achieve much higher peak flow rates during filling than conventional techniques. Published in the CIRP Journal of Manufacturing

Science and Technology, Drummer and Vetter’s paper compared the morphology and mechanical properties of scaled-down versions of ISO test pieces at 1:8 and 1:16 (equivalent to thicknesses of 0.5mm and 0.25mm) moulded in commercial polypropylene and acetal grades using both conventional injection techniques and an X-melt type expansion method. Polarised light microscopy and Differential Scanning

Calorimetry (DSC) was used to inspect and determine the degree of crystallinity in the parts, revealing a thinner and more highly oriented skin layer with a smoother core morphology comprised of smaller spherulites in the expansion injection moulded parts produced in both PP and acetal. Mechanical properties were measured using static tensile and Dynamic Mechanical Analysis (DMA) and were somewhat contrasting. Static tensile tests recorded no significant difference in tensile modulus for expan- sion and conventional injection moulding; DMA results showed a slightly lower

tensile modulus for the expansion moulded parts, which the researchers suggest is the result of the thinner skin layer and different spherulitic structure in the core. ❙

Above: X-melt expansion moulding set up for multi- cavity hot runner use with valve gates and

simultaneous needle


Left: A medical sleeve weighing 1.6g produced in PP using Ferroma- tik’s HiPM technique

April 2012 | INJECTION WORLD 33

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